1. Field of the Invention
The invention relates generally to the field of post tension systems for reinforcing concrete structures. More particularly, the invention relates to anchors for post tension cables.
2. Background Art
For quite some time, the design of concrete structures imitated typical steel structure designs of columns, girders and beams. With technological advances in structural concrete, however, designs specific to concrete structures began to evolve. Concrete has several advantages with respect to steel, including lower cost, not requiring fireproofing, and having plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive (vertical) load, is essentially unable to carry significant tensile loads. In order to enable concrete structures to carry tensile loads, it is necessary, therefore, to add steel bars, called reinforcements, to the concrete. The reinforcements enable the concrete to carry the compressive loads and the steel to carry the tensile (horizontal) loads.
Structures made from reinforced concrete may be built with load-bearing walls, but this configuration does not use the full potential of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economical and popular method of building concrete structures. Reinforced-concrete framing appears to be a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. Steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required compressive strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs for concrete frames is the beam-and-slab. The beam and slab system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be used over and over for the same shape. The beam and slab system, therefore, produces an economically advantageous structure.
With the development of flat-slab construction, exposed beams can be eliminated. In the flat slab system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions. Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as 100 feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength steel are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing is held loosely in place while the concrete is placed around it. The reinforcing is then stretched by hydraulic jacks and securely anchored into place. Prestressing is performed with individual members in the shop and post-tensioning is performed as part of the structure on the construction site. In a typical tendon tensioning anchor assembly in such post-tensioning operations, there is provided a pair of anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
One such post tensioning system is described in U.S. Pat. No. 3,937,607 issued to Rodormer. The general principle is explained with respect to FIG. 3 in the '607patent and states, in relevant part, “[i]n accordance with conventional techniques, a center hole electro-hydraulic jack is placed on each tendon to tension the tendon. When the jack is released the live end anchor chuck 40 will set and grip the tendon holding the latter at the desired tension.” The chuck, or retaining wedge, known in the art is typically a conical-exterior shaped insert which fits in a mating, tapered opening in an anchor plate. The chuck or wedge may be divided into two or more circumferential segments to enable application to the exterior of the tendon or cable prior to insertion into the opening in the anchor plate. The interior opening of the chuck typically includes conventional buttress threads in order to deform and thus grip the exterior surface of the tendon or cable, such that when the jack or tensioning device is released, the tension in the tendon will be transferred to the chuck, and thus to the anchor plate.
Recently, certification procedures for the tensile strength of post tensioning devices promulgated by the American Society for Testing and Materials (ASTM) were amended to provide a standard for the absolute ultimate tensile strength (AUTS) of post tensioning devices. As a result of the new certification procedures, it has been determined that post tensioning devices made using tendon steel compositions and configurations known in the art fail certification testing in a substantial number of cases. The steel alloys used in post tensioning devices are already developed to such an extent that improving the tensile strength of the tendons themselves would be difficult and expensive. Accordingly, there is a need for a configuration of a post tensioning anchor system which has improved tensile strength using materials known in the art, and while maintaining the dimensions of post tensioning anchor systems known in the art.